This invention relates to a pincushion correction circuit having adjustable control over the amount of pincushion correction.
Pincushion raster distortion can be corrected by using a saturable reactor or transformer to generate a variable reactance having a parabolic shape. In many circuits, a winding of the saturable transformer is placed in series with one of the deflection coils to modulate the deflection current with a parabolic signal derived from the deflection signal for the other deflection coil. When east-west or side pincushion distortion is being corrected, horizontal drive current to the horizontal drive coil is modulated with a vertical parabolic signal. In a saturable reactor type pincushion correction circuit, some degree of variable adjustment has been possible, as shown for example in Lempke U.S. Pat. No. 3,329,859, Barkow et al. U.S. Pat. No. 3,329,861 and Lempke U.S. Pat. No. 3,329,862.
Pincushion correction circuits have also been developed which do not use a saturable reactor or transformer but rather depend on a different type of circuit to introduce a parabolic curve to the deflection waveform. For example, an integrator may be used to produce a parabolic waveform which is inductively coupled through a nonsaturable transformer to superimpose the correction current on the deflection signal, as shown for example in Knorr U.S. Pat. No. 3,452,243.
The amount of pincushion correction necessary at the beginning and end of scan may be unequal. In side pincushion distortion, the amount of curvature or bowing at the "east" side of the raster may be different than the amount of bowing at the "west" side of the raster. In some cases, the amount of bowing may be inversely proportional, but in other cases, the amount of bowing may not be related. In prior pincushion correction circuits using a saturable reactor, separate and/or independent adjustment of the amount of correction at the beginning and end of trace has not been possible, or has required unduly complex circuitry.